Add a huge bunch of docstrings

lib/tfw/mixins/callback_mixin.py

@@ -9,6 +9,12 @@ class CallbackMixin:
         self._callbacks = []
 
     def subscribe_callback(self, callback, *args, **kwargs):
+        """
+        Subscribe a callable to invoke once an event is triggered.
+        :param callback: callable to be executed on events
+        :param *args: arguments passed to callable
+        :param **kwargs: kwargs passed to callable
+        """
         fun = partial(callback, *args, **kwargs)
         self._callbacks.append(fun)